1. Field of the Invention
This invention relates to contacting a hydrocarbon-containing gas stream with a physical solvent and particularly relates to contacting a hydrocarbon gas stream with a preferential physical solvent. It more specifically relates to separating and recovering ethane and higher boiling hydrocarbons from a hydrocarbon-containing gas stream and especially relates to simplification of the Mehra Process by elimination of the flashing step.
2. Review of the Prior Art
Hydrocarbons must often be recovered from such gas streams as hydrocarbon gas, alkylates, reformates, and the like. Many recovery processes are available, but countercurrently contacting the upwardly flowing gas stream with a downwardly flowing liquid under conditions furnishing high interfacial surface area is often a preferred recovery process, known as absorption or extraction.
Most physical solvents show some preference among hydrocarbons in a mixture thereof. In other words, they have greater solvency, perhaps because of a stronger physical attraction, for one or more hydrocarbons in such a mixture. This preference is measured by the absorption principle, leading to an alpha or relative volatility. Most of the commonly used lean oils, for example, have relative volatilities of methane over ethane of slightly less than 5.
Lean oils have been used in absorption plants for extracting C.sub.4 + hydrocarbons, with some recovery of propane, from hydrocarbon gas streams for many years. The lean oils are non-selective for lighter hydrocarbons, such as ethane and propane, so that relatively large amounts of methane are abosrbed, thereby making the separation of ethane and propane from methane quite difficult and expensive. Due to the market demand for lighter hydrocarbons, such as ethane and propane, and the lack of selectivity of lean oils for such components, the absorption processes have been replaced by processes consisting of refrigerated oil absorption, simple refrigeration, cascaded refrigeration, Joule-Thompson, or cryogenic expander processes. The related Mehra Process patents and applications are directed toward physical solvents having strongly preferential characteristics. The Mehra Process thereby overcomes the disadvantages of non-selectivity of common lean oils for lighter hydrocarbons, such as ethane and propane.
Furthermore, the recovery levels of various hydrocarbons from the above processes used for the extraction of C.sub.2 + hydrocarbons are quite inflexible. The Mehra Process overcomes the inflexibility drawback by effectively utilizing the selectivity characteristics of preferential physical solvents. Typical recoveries for these processes are compared in Table I.
TABLE I ______________________________________ COMPARISON OF TYPICAL LIQUID RECOVERIES PRO- BU- GASO- ETHANE PANE TANES LINE EXTRACTION (%) (%) (%) (%) ______________________________________ ABSORPTION 4 24 75 87 REFRIGERATED AB- 15 65 90 95 SORPTION SIMPLE REFRIGER- 35 80 93 97 RATION CASCADED RE- 70 96 99 100 FRIGERATION JOULE-THOMPSON 75 96 99 100 EXPANSION TURBO-EXPANDER 85 97 100 100 MEHRA PROCESS 2-98 2-99 2-100 100 ______________________________________
In summary, the oil absorption, refrigerated oil absorption, simple refrigeration, and cascaded refrigeration processes operate at the pipeline pressures, without letting down the gas pressure, but the recovery of desirable liquids (ethane plus heavier components) is poor, with the exception of the cascaded refrigeration process which has extremely high operating costs but achieves good ethane and propane recoveries. The Joule-Thompson and cryogenic expander processes achieve high ethane recoveries by letting down the pressure of the entire inlet gas, which is primarily methane (typically 80-85%), but recompression of most of the inlet gas is quite expensive. The Mehra Process combines the advantages of the higher-pressure extraction processes by selectively recovering and letting down the pressure of essentially the desired components, thereby reducing the compression of undesirable components, such as methane, while achieving high levels of component recovery in a flexible manner.
In all of the above processes, except the Mehra Process, the ethane plus heavier components are recovered in a specific configuration determined by their composition in the raw hydrocarbon gas stream and equilibrium at the key operating conditions of pressure and temperature within the process.
Under poor economic conditions when the ethane price as petrochemical feedstock is less than its equivalent fuel price and when the propane price for feedstock usage is attractive, the operator of a hydrocarbon gas liquid extraction plant is limited as to operating choice because he is unable to minimize ethane recovery and maximize propane recovery in response to market conditions.
The refrigeration process, which typically recovers 80% of the propane, also typically requires the recovery of 35% of the ethane. In order to boost propane recovery to the 95+% level, cascaded refrigeration, Joule-Thompson, or cryogenic turbo-expander processes would have to be used while simultaneously boosting the ethane recovery to 70+% at a considerably larger capital investment.
The parent patents and applications related to the Mehra Process have utilized preferential physical solvents for recovering hydrocarbon gas liquids from hydrocarbon gas streams by extracting the hydrocarbon gas streams with a preferential physical solvent, flashing the rich solvent, and compressing, cooling, and condensing the desired C.sub.2 + hydrocarbons. In carrying out the extraction of desired hydrocarbons, several streams had to be recycled, thereby requiring accessory equipment, such as compressors, coolers, condensers, associated piping, automatic control valves, pressure gauges, and data recording equipment. Furthermore, flashing of the rich solvent stream was carried out in multiple steps, consistent with economic criteria involving energy consumption and capital investment. Even though the energy consumption was lower for the Mehra Process than for conventional state-of-the-art processes, several steps were required that increased the complexity and overall capital requirements of such a plant. There is consequently a need for simplification of the Mehra Process in order to reduce its capital investment requirement.
U.S. Pat. No. 3,287,262 describes a process for contacting and recovering hydrocarbons such as hydrocarbon gasoline from raw or wet hydrocarbon gas. A lean oil which has been presaturated with ethane passes through an absorber countercurrently to a hydrocarbon gas stream to produce a rich oil which is sent to a de-ethanizer comprising a kettle section, a stripping section, and an absorbing section. From the bottom of this de-ethanizer, a kettle product or rich oil containing C.sub.3 -C.sub.6 hydrocarbons is produced. From the top thereof an ethane stream is produced which is sent to a presaturator column in which lean oil passes downwardly to become presaturated with the ethane and form the ethane-presaturated feed for the absorber.
U.S. Pat. No. 3,520,143 shows in its FIG. 4 a washing or absorption process for purifying methane contaminated with ethane or ethylene. Propane, as the extraction liquid, is stripped in a stripping columr with heat to produce a stream of higher boiling hydrocarbons, such as ethane and ethylene, which is sent to a rectification column which also receives the liquefied gas mixture.
U.S. Pat. No. 3,607,734 is directed to the use of an integrated column for absorbing, demethanizing (or de-ethanizing or depropanizing), and splitting operations. Gaseous feed is then introduced to the absorber section, and liquid feed is then introduced to the absorber section and/or the splitter section. A light naphtha fraction from the splitter section is used as the primary absorbing medium in the absorber section. Heavy naphtha, also from the splitter section, is used as lean oil in a sponge section, at the very top of the column, primarily to remove light lean oil from the absorber section overhead. Absorber fat oil, light naphtha, heavy naphtha, bottoms, and fuel gas are or can be produced as products. This integrated column is intended to replace conventional process designs in which gas and liquid are fed to an absorber, overhead is fed to a sponge tower, and bottoms are fed to a splitter which produces overhead and bottoms. The splitter overhead is fed to a debutanizer which produces C.sub.4 and lighter as overhead, light naphtha as an intermediate fraction, and intermediate naphtha as a bottom fraction. The bottoms from the splitter overhead are sent to a rerun tower which produces heavy naphtha as overhead and heating oil or catalyst feed as the bottoms fraction.
U.S. Pat. No. 4,191,640 is directed to a dual fractionating process for hydrocarbons which employs high pressure and low pressure fractionating zones. A stable hydrocarbon stream, such as propane, is introduced into one of the fractionating zones, this hydrocarbon stream being stable under the conditions existing in the high pressure fractionating zone and at least a portion of this hydrocarbon stream having a boiling point lower than the boiling point of at least one of the unstable components which accumulates in the high pressure fractionating zone. This process is directed to removing accumulations of unstable components in the lower section of the high pressure fractionating zone in the column. These unstable components are generally highly unsaturated in nature, e.g., acetylenes, dienes, and the like. If an ignition source is present, such as static electricity, detonation of these unstable components can occur.
In U.S. Pat. No. 4,150,962, high pressure hydrocarbon gas is sweetened with lean amines, dried with ethylene glycol, reduced in pressure in a turbo expander, scrubbed with a C.sub.4 rich liquid, dried with a solid desiccant, and liquefied to form liquid methane. The rich stream from the scrubber is stripped in a fractionating column to remove methane, then stripped in another fractionating column to remove ethane, next stripped in a third fractionating column to remove propane, and then sent to a splitter to remove butane as overhead and C.sub.5 + hydrocarbon as bottoms.